The present invention relates to a ladder-type electric filter apparatus for a variety of radio equipments.
It is generally known that such a filter apparatus comprises at least one electric filter unit each of which includes one or more series piezoelectric resonator elements, one or more parallel piezoelectric resonator elements, terminal plates each having a connecting leg and a casing for containing the resonator elements and the terminal plates. It is also known that a filter circuit is normally constructed by successively connecting a plurality of such electric filter units.
Recently, there is an increasing demand for smaller and thinner dimensions of the radio equipment incorporating such a filter apparatus. In order to satisfy this demand, it has been attempted to possibly decrease the height of the filter arrangement by containing only one or a few essential filter units within one casing.
With the conventional electric filter apparatus, each filter unit is mounted on a printed circuit board, connecting terminal legs extended from the casing of each unit are bent toward the printed circuit board and are connected to predetermined conductive lines provided on the board. The connection of each connecting leg to the circuit board is performed at only one side of the casing. That is, each of the connecting terminal legs is extended from only one side wall of the casing and connected to the circuit board by soldering means or inserting it to an associated through hole. However, the fixing of each filter unit to the circuit board does not have sufficient strength because each filter unit is fixed to the circuit board by a cantilever type supporting or one side supporting. Consequently, if the apparatus receives a shock or an impact, the fixed portion may be sprung loose so that the apparatus becomes unstable, or the filter unit may be disconnected from the printed circuit board.
With a view to overcoming these disadvantages, there has been proposed in Japanese U.M. Application No. 63-110142 a filter arrangement in which the connecting legs of a plurality of terminal plates in each filter unit are protruded from the opposite sides of a casing. With this arrangement, since each filter unit is supported at both sides of the casing by soldering the respective connecting legs to a printed circuit board or inserting them into through holes provided on the board, each filter unit can be steadily sustained.
However, in order to extend the connecting legs of the terminal plates, it is generally considered to be necessary to use a casing having openings at both sides. Upon assembling of the filter unit, after a cover plate having one or more holes for passing one or more associated connecting legs of one or more terminal plates is fitted into one opening of the casing, said one or more terminal plates are inserted through the other opening at the opposite side into the casing with the connecting leg(s) thereof being passed through the associated hole(s) in the fitted cover plate, and then each resonator and the remaining terminal plate(s) are inserted into the casing. After the insertion of the components is completed, another cover plate is fitted into the other opening of the casing by insertion of the connecting leg(s) of the remaining terminal plate(s) into one or more holes provided on this cover plate. Finally, filler material is applied to each cover plate.
Furthermore, with this previously proposed arrangement, since the terminal plates are to be inserted into the casing from one side thereof, it is necessary to mount the terminal plate(s) to be first inserted into the casing while the connecting leg(s) of the terminal plates are passed through the hole(s) of the cover plate precedingly fitted into one or opposite opening. It is difficult to insert the connecting leg(s) of the terminal plate(s) from the other side opening into the hole(s) of the cover plate on one opening because each connecting leg may be easily deformed during this procedure. Therefore, this previously proposed arrangement is complicated in its assembling and thus unsuitable for mass-production. Also, since the casing is provided with the openings at the both sides, not only sealing for the openings may be troublesome, but also it may be difficult to obtain full tightness in each sealing portion.